Effect of metamorphic reaction history on the U-Pb dating of titanite

Drill core samples of garnet-clinopyroxene granulite at Tirschheim and a reference sample at Waldheim (Saxon Granulite Massif, Germany) endured the same P-T conditions, but developed variable mineral assemblages due to differences in bulk chemistry, reaction progress, deformation and retrogression. Titanite formed during peak-metamorphic conditions of 22–24 kbar and 1020–1050 °C. Dating titanite from the various samples should yield the same age for all. The observed age variation, which exceeds the duration of the entire metamorphic cycle, originates from the contrasting preservation of isotopic inheritance during peak metamorphism and from post-peak re-equilibration. (1) Pb inheritance observed in some peak-metamorphic titanite demonstrates that geochronologically relevant elements are redistributed among remaining reactants and reaction products during prograde metamorphism and that the sequence of metamorphic reactions does not result in isotopic homogenization. Instead, metamorphic minerals inherit the radiogenic signatures of the precursor minerals and may in extreme cases approach the age of the precursor mineral. (2) Titanite that formed at peak-metamorphic conditions is characterized by high A1 contents and XF ≈ 0.8−1. Texturally comparable titanite that re-equilibrated during cooling (reduced Al contents and XF) yields too young U-Pb ages. The age of such re-equilibrated titanite does not correspond to the age of the event indicated by the texture.

采自蒂尔施海姆（Tirschheim）的石榴子石-单斜辉石麻粒岩（garnet-clinopyroxene granulite）岩心样品，以及采自德国萨克森麻粒岩地块（Saxon Granulite Massif）瓦尔德海姆（Waldheim）的参考样品，尽管经历了相同的压力-温度（P-T）条件，但由于全岩化学组成、反应进程、变形作用及退变质作用的差异，形成了各异的矿物组合。榍石（Titanite）形成于22–24千巴、1020–1050 ℃的峰期变质条件下。对不同样品中的榍石开展定年工作，理论上应获得一致的年龄结果。但实际观测到的年龄差异远超整个变质旋回的持续时长，其成因在于峰期变质过程中同位素继承作用的保存程度存在显著差异，以及峰期后的再平衡作用。
(1) 部分峰期变质榍石中观测到的铅（Pb）继承现象表明，在进变质作用过程中，与地质年代学相关的元素会在剩余反应物与反应产物之间发生重新分配，且变质反应序列并未实现同位素均一化。反之，变质矿物会继承前驱矿物的放射成因同位素特征，在极端情况下甚至可趋近于前驱矿物的形成年龄。
(2) 形成于峰期变质条件下的榍石具有高铝（Al）含量与X_F≈0.8−1的特征。而在冷却过程中发生再平衡的结构可比榍石（铝含量降低且X_F发生变化），其测得的U-Pb年龄会偏年轻。这类经再平衡的榍石的年龄，并不对应其结构所指示的地质事件的形成年龄。